The Geological Story of the Arnside & Silverdale Area of Outstanding Natural Beauty
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The Geological Story of the Arnside & Silverdale Area of Outstanding Natural Beauty Bibliographic Reference: Thompson, A and Poole, J.S. 2019 (b): The Geological Story of the Arnside & Silverdale AONB. Cuesta Consulting Limited, East Lambrook. QA Reference: C/ASAONB/021. Issued 18th January 2019 Photographic Acknowledgements: All photographs used in this report were taken by the Authors and used with their permission. In accordance with clause 2.15 of the contract dated 7th March 2018 between the Arnside and Silverdale AONB Partnership and Cuesta Consulting Ltd, except where otherwise stated, those photographs are the copyright of the Arnside and Silverdale AONB Partnership. The GEOLOGICAL STORY of the ARNSIDE & SILVERDALE AONB CONTENTS Introduction .................................................................................................... 2 Geo-Jargon - A few essential terms explained ............................................................ 3 Geodiversity ................................................................................................................ 4 The Geological Timescale ............................................................................................ 6 Before the Rocks were Made .......................................................................... 7 Shallow Seas and Advancing Deltas – the Carboniferous Period ...................... 8 The Carboniferous Limestone Sequence ...................................................................................... 9 Cracking the Crust – the Variscan Orogeny .................................................... 13 The Deserts Return – the Permian and Triassic Periods ................................. 16 Sunk without Trace – the Jurassic and Cretaceous Periods ............................ 16 Dissolving Limestone and Further Collisions – the Tertiary Period ................. 17 Karstic Features of the Limestone Pavements: Clints, Grikes, Runnels & Kamenitzas ................. 18 Larger-scale Karst: Limestone Pavements, Cave Systems, Dolines and Poljes ............................. 19 Climate Change and Landscape Response – the Quaternary Period............... 20 Pleistocene – the Quaternary Ice Ages ..................................................................... 23 Hawes Water: A Natural Archive of Quaternary Climate Change ............................................... 27 Holocene – Global Warming, Rising Seas and the Growing Influence of Man .......... 30 The Present-Day Landscape and Seascape ............................................................... 39 Further Reading ............................................................................................ 41 1 The GEOLOGICAL STORY of the ARNSIDE & SILVERDALE AONB Introduction The highly varied landscape of the Arnside & Silverdale Area of Outstanding Natural Beauty (AONB) owes its origin to nearly half a billion years of geological history and evolution. It is an intricate, close-knit landscape that has been shaped, primarily, by its geology and by the range of natural processes which have acted upon it over unimaginable periods of time. Understanding how the various rocks within the area were formed, and how they have since been sculptured by ice, wind, rain and rivers, provides a fascinating insight into the nature of the area and the reasons behind its remarkable variety. The underlying geology, topography and diversity are reflected in the wide range of special (‘priority’) habitats which have developed in this area over the last 10,000 years or more, and which now contribute further to its natural heritage. In more recent times, generations of farming, tourism, built development and other land uses have also left their mark on the landscape, but in many cases, even these reflect the underlying geology and landforms of the area. 2 The GEOLOGICAL STORY of the ARNSIDE & SILVERDALE AONB Geo-Jargon - A few essential terms explained This booklet is intended primarily for those who are interested in the origins of the landscape and features of the AONB but who do not, necessarily, have any prior geological knowledge. It aims to do so without the use of unnecessary technical terms, so that anyone can follow the geological ‘story’ of the area without needing to be an expert. In short, it seeks to explain things, as far as possible, in ‘Plain English’. There are, however, a small number of geological terms which are difficult to avoid and very useful to understand. The main ones are explained briefly below, and a few others are explained as and when they are first used, in the text. Geology is simply the scientific study of the Earth. It encompasses the study of all rocks, superficial deposits, minerals and fossils, including how they were formed, how old they are, and what evidence they provide about the conditions which prevailed in a particular area at the time of their formation. Rocks are the solid materials which make up the surface of the Earth and much of its internal structure. They include igneous rocks (formed by the solidification of molten material either deep inside the Earth or, in the case of ‘volcanic’ rocks, ejected at the surface through volcanoes and fissures); sedimentary rocks (formed either by the erosion of older rocks, and the subsequent deposition of the eroded material into new layers, or by the accumulation of shells and other organic remains, usually in water); and metamorphic rocks (those which have been changed from their original form, either by high temperatures and/or pressures within the Earth’s crust). Superficial deposits are sediments which have been deposited on the surface of the Earth, generally within the last 2 million years, and which have not yet become hardened into rocks. Minerals are the inorganic constituents of rocks and sediments, occurring as individual crystals (in the case of igneous and metamorphic rocks) or eroded grains of sand etc. (in sedimentary rocks). Individual minerals are distinct chemical compounds, such as Calcium Carbonate (or Calcite, the main constituent of limestone), or Silicon Dioxide (Silica, the main constituent of most sandstones). Rocks may comprise just one or (usually) more than one type of mineral. Fossils are the preserved traces or remains of animals and other organisms which formerly inhabited the Earth. In most cases, it is only the shells or skeletons which are preserved, often as a result of the recrystallisation or even replacement of the original minerals that were present within the structures. Geomorphology literally means the scientific study of the shape of the Earth’s surface features (landforms), and of the natural geomorphological processes which have been responsible for their formation. These include the processes of weathering, erosion, sediment transportation and deposition. They may involve rivers, glaciers, landslides, rockfalls, mudflows, tidal currents and wind, along with mechanical effects of temperature variations and the chemical dissolution or precipitation of minerals in water. Karst, and ‘karstic’ are more specialised terms which have particular relevance to areas, such as the Arnside & Silverdale AONB, where limestone rocks are exposed at the surface. They relate to the special landforms and processes which are associated with the natural dissolution of limestone by water. Further details of these are given on pages 18 and 19 below. 3 The GEOLOGICAL STORY of the ARNSIDE & SILVERDALE AONB Geodiversity In addition to the technical terms explained above, it is worth understanding the importance of one other, and that is ‘geodiversity’. This is a term which is used to describe the sheer variety of all geological and geomorphological features (and processes) which can be seen within a given area. It is the geological equivalent of ‘biodiversity’, which covers the variety of living things, and is a key aspect of geological conservation. In the case of the Arnside & Silverdale AONB, the rocks themselves (almost entirely of limestone) are quite limited in their variety – at least to the casual observer – but the detailed evidence which they provide regarding past environments, and the intricate variety of structures, features and landforms which have been created over time through the action of natural processes upon these rocks, is quite astonishing for such a small geographical area. Some, though by no means all, of this geodiversity is reflected in a range of environmental designations (protected areas) within the AONB which relate (in part, at least) to geological or geomorphological features. These range from Local Geological Sites (LGS), proposed by the local geo-conservation groups (Cumbria Geo-Conservation and Geo-Lancashire) and designated by the County Councils, and Local Nature Reserves (LNRs) to nationally-designated Sites of Special Scientific Interest (SSSIs), National Nature Reserves (NNRs) and Limestone Pavement Orders (LPOs). Map 1, on the following page, attempts to capture this diversity and importance by combining information on the distribution of different rock formations and superficial sediments in the area with the distribution of these geologically-influenced designations. Other maps, presented later in the booklet, show the bedrock formations and superficial deposits separately, and the accompanying text explains each of the categories along with other features shown on the map. It should be noted that other environmental designations, relating primarily to biological characteristics (vegetation, wildlife and habitats) also occur within